It is widely accepted that the Munsell notation system of numerical values for designating a color is a most excellent system for expressing the color of an object because of the high correspondence of the values according to the Munsell system and the luminosity of the color of the object. However, it is not very easy to convert the tristimulus values X, Y and Z according to the CIE notation system, which are calculated on the basis of a colorimetric value, into a value according to the Munsell notation system.
The Optical Society of America considered the relation between the values according to the CIE notation system and the values according to the Munsell notation system, and in 1943 proposed a revised Munsell notation system, known as the Munsell Renotation System, but hereinafter referred to simply as the Munsell notation system. According to this system, the relation between the visual reflection power Y(%) and Munsell lightness V is determined by using a quinary formula experimentarily obtained. For the conversion of the chromaticity values according to the CIE notation system, an equi-hue locus and an equi-chroma locus must be found on a chromaticity-Munsell notation conversion diagram, which is a CIE 193 (x, y)--chromaticity diagram corresponding to Munsell lightness value V close to the value V for the particular color as converted from the CIE notation system, and which has Munsell equi-hue lines and equi-chroma loci superimposed therein, from which the Munsell values for hue and chroma as marked on the diagram can be determined.
This conversion process is described in ASTM Designation D 1535-80 Standard Method of SPECIFYING COLOR BY THE MUNSELL SYSTEM. Using this process, it is fundamentally possible to convert a value according to the CIE notation system into a value according to the Munsell notation system.
However, the relation between a value according to the CIE notation system and a value according to the Munsell notation system can be expressed by a numerical formula only for the relation between the visual reflection power Y and the Munsell lightness V. Therefore, in order to convert a value according to the Munsell notation system, it is necessary to carry out the troublesome steps of using the chromaticity-Munsell notation conversion diagrams.
For this reason it is thought that the values according to the Munsell notation system have not been widely utilized, although they have a high correspondency to the luminosity of colors.
Recently, however, the complexity of the conversion because of the necessity of using the chromaticity-Munsell notation conversion diagrams have been overcome by the utilization of computers, so that it has become possible to carry out the conversion very easily and speedily. Therefore, it is possible that the Munsell notation system will be utilized to a greater extent in the future.
However, the tables of numerical values representing the relation between the values of visual reflection power according to the CIE notation system and the lightness values according to the Munsell notation system and the chromaticity-Munsell notation conversion diagrams proposed by the Optical Society of America do not fully cover the whole range of colors of objects. They cover only a limited range of colors of objects. This has been an obstacle to the utilization of the values according to the Munsell notation system.
The values according to the CIE notation system of certain colors, especially those having high visibilities, are substantially incapable of being converted into values according to the Munsell notation system by the system presently available.
The above drawbacks of the Munsell notation system have not heretofore posed any big problems. This may be due to the fact that the Munsell notation system has not been utilized very much in practice.
There has also been developed a new color-expressing notation system for colored materials, namely the HC*B* system, which is based on and is derived from the Munsell notation system.
In the HC*B* notation system, the values designating colors are determined on the basis of hue H, lightness V and chroma C according to the Munsell notation system, and the system is described in Japanese Patent Application No. 205282/1981, "Apparatus for Measuring and Displaying Values C* of Density and Degrees of Fastness of Colors".
In order to be able to make sufficient use of not only the values according to the Munsell notation system but also those according to the HC*B* notation system, it is very inconvenient that regions in the range of colors exist in which it is substantially not possible to convert, by the system presently available, values according to the CIE notation system of colors of objects into values according to the Munsell notation system.